Type classes in GADTs

Thanks for the explanation. I see how this wouldn't behave nicely with automatic class constraint inference. I didn't test the example on any other GHC versions.
I will probably end up passing in the Eq dictionary from outside like Daniil suggested. I would prefer to do the following, but GHC doesn't accept the type signature.
evidenceOfEq :: CAOp a -> (Eq a => b) -> b
Neither does it accept data EqConstraint a b = EqConstraint (Eq a => b). Foiled again.
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